1. Field of the Invention:
This invention relates to a molded article of methacrylic resin excelling in transparency, weatherability, and abrasion resistance and possessing high heat resistance and to a method for the production thereof.
2. Description of the Prior Art:
Methacrylic resins excel in appearance, transparency, and weatherability and, therefore, find extensive utility in indoor and outdoor applications to glazings, lampshades, decorations, etc. They, however, are inferior in heat resistance to inorganic glass and are further deficient in scratchproofness. When they have these qualities improved, they can be expected to attain further growth in utility. Various studies are now under way in search of means of mending such drawbacks as mentioned above. Generally, there prevails a practice of forming synthetic resin molding materials by the use of various molding machines such as press molding machine, injection molding machine, extrusion molding machine, and vacuum molding machine or by the polymerization and formation in casting dies and then applying abrasion resistant film-forming materials on the surface of the resulting molded articles by the technique of spraying or dipping and polymerizing and hardening the applied layers of the film-forming materials by means of electron beam or heat thereby covering the molded articles with abrasion resistant thin films. The problem encountered by this method resides in the fact that since the step of polymerization for the formation of the abrasion resistant thin film is carried out subsequent to the step of molding, such expensive facilities as those for coating, curing, and dust removing must be disposed separately of those used for the molding and the operation of this method entails complicate management such as of the viscosity of the film-forming materials, for example.
As means of improving the method just described, Japanese Patent Publication SHO 54(1979)-14,617 and SHO 54(1979)-14,618 disclose a method for producing a molded article of synthetic resin possessing an abrasion resistant surface, which comprises applying an abrasion resistant film-forming material on a mold, polymerizing the material as covered with a synthetic resin film or kept under an atmosphere of inert gas, relieving the material being polymerized of the film or the inert gas after the gel content has reached a level in the range of about 40 to 95%, post-curing the polymerized material generally until the gel content reaches a level not lower than 85% thereby allowing the material to assume a state neither swelled nor dissolved by the basic resin material to be poured therein afterward, pouring the basic resin material into the mold, and polymerizing and forming the resin material. This method, however, entails inevitable complication of process because the impartation of abrasion resistance to the required two steps of polymerization, i.e. prior polymerization and post polymerization. In this case, when the prior polymerization is exclusively used, the produced molded article acquires a defective appearance because the surface thereof has craters and cracks, shows uneven abrasion resistance, and fails to exhibit ample abrasion resistance.
As a second problem, insufficient adhesiveness between the basic resin and the abrasion resistant thin film can be cited. As means of solving this problem, there exists a method which resorts to interposing between the abrasion resistant thin film and the basic resin a layer capable of adhering to both the outer layers. This method suffers from poor productivity because it requires the step of application and the step of polymerizing the applied layer to be repeated at least twice. The molded article obtained by this method has a disadvantage that since pinholes due to the contamination of dust existing in the abrasion resistant film is inevitable and tends to allow the adhesive layer to be dissolved at those points, the parts of the coat centering around the pinholes as the cores suffer from notable impairment of the resistance to solvent.
Japanese Patent Publication SHO 53(1978)-9,876 discloses a method which effects improvement of adhesiveness by exposing the surface of a molded article to ultraviolet light and applying a abrasion resistant thin film on this surface of the molded article. This method, however, is complicate in process because it requires both the molded article and the film formed thereon to be separately exposed to ultraviolet light. The abrasion resistant thin film formed by this method on the molded article has been demonstrated by a QUV weathering test to have not served the purpose of improving the abrasion resistance of the molded article. There also prevails a practice of adding to the raw material for the abrasion resistant thin film a good solvent for the basic molding resin. If this power is too high, this practice has the dissibility of inflicting cracks on the surface of the molded article or impairing the smoothness of the surface. Conversely, if the power is weak, the desired tight adhesion is not obtained. Thus, this practice entails complication in the formulation of the solvent and, at the same time, requires installation of facilities exclusively for disposal of the waste gas resulting from the vaporization of the solvent, for example.
Methacrylic resins using methyl methacrylate as a main component possess excellent weatherability and outstanding transparency and, therefore, have found utility in applications to lampshades and automobile parts. These methacrylic resins, however, are linear polymers and, therefore, do not suit applications which demand relatively high heat resistance, resistance to solvents, resistance to shocks, and surface hardness. In terms of heat resistance, they cannot be used in applications requiring protracted exposure to elevated temperatures exceeding 100.degree. C., such as those to head lamp lens for automobiles, covers for solar heat warmers, etc.
Heretofore, for the purpose of improving heat resistance of methacrylic resins, there have been introduced numerous methods such as, for example, a method which relies on copolymerization to methyl methacrylate with .alpha.-methyl styrene (U.S. Pat. No. 3,135,723), a method which resorts to copolymerization of methyl methacrylate with .alpha.--methyl styrene and maleic anhydride (Japanese Patent Publication SHO 45(1970)-31,953 and Japanese Patent Publication SHO 49(1974)-10,156), and a method which adopts copolymerization of methyl methacrylate with .alpha.--methyl styrene and maleimide (Japanese Patent Laid-Open SHO 48(1973)-95,490).
Although these methods are invariably capable of improving heat resistance, some of them suffer from notably low rates of polymerization and fail to attain high ratios of polymerization and some others, though capable of producing polymers efficiently in relatively short spans of time, suffer the produced polymers to assume colors strongly and degrade in transparency, weatherability, surface hardness, mechanical strength, etc.
It has been separately proposed to produce a molded article using a gel polymer which is obtained by partially cross-linking an alkyl methacrylate homopolymer or syrup with a cross-linking agent (Japanese Patent Laid-Open SHO 60(1985)-212,128). This method is capable of producing a cross-linked molded article excelling in transparency and 7 heat resistance. When an abrasion resistant thin film is applied on this molded article, there ensues a disadvantage that this molded articles exhibits poor adhesiveness to the abrasion resistant thin film. For effective application of this film on the surface of the molded article, it becomes necessary to enhance the adhesiveness of the film by subjecting this molded articles to a complicated preparatory treatment as mentioned above.
An object of this invention, therefore, is to provide a molded article of methacrylic resin excelling in both abrasion resistance and heat resistance, and more particularly a molded article of methacrylic resin having a abrasion resistant thin film adhere fast thereto and enjoying high heat resistance.
Another object of this invention is to provide a method for easy production of a molded article of the foregoing description.